6/09/2015

New Giant Volcano Below Sea Is Largest in the World

A 3-D map of the Tamu Massif formation, which scientists now say is one huge shield volcano Illustration courtesy IODP

A volcano the size of New Mexico or the British Isles has
been identified under the Pacific Ocean, about 1,000 miles (1,600
kilometers) east of Japan, making it the biggest volcano on Earth and
one of the biggest in the solar system.

Called Tamu Massif, the giant shield volcano had been thought
to be a composite of smaller structures, but now scientists say they
must rethink long-held beliefs about marine geology.

“It is in the same league as Olympus Mons on Mars, which had been considered to be the largest volcano in the solar system,” Sager told National Geographic.

Tamu Massif is a rounded dome that measures about 280 by 400 miles
(450 by 650 kilometers), or more than 100,000 square miles. Its top lies
about 6,500 feet (about 2,000 meters) below the ocean surface, while
the base extends down to about 4 miles (6.4 kilometers) deep. Tamu
Massif dwarfs the largest active volcano on Earth, Mauna Loa in Hawaii,
which measures about 2,000 square miles (5,200 square kilometers).

Made of basalt, Tamu Massif is the oldest and largest feature of an
oceanic plateau called the Shatsky Rise in the northwestern Pacific
Ocean. The total area of the rise is similar to Japan or California.

Sager started studying Tamu Massif about 20 years ago. He named it
Tamu Massif because Tamu is short for Texas A&M University, where
the scientist worked at the time; massif is French for “massive” and is a scientific term for a large mountain.

Scientists had known about the Shatsky Rise since the early 20th
century, when it was first mapped, he explained. “We knew it was a big
mountain range, but we didn’t know what the structure was like or how it
formed,” said Sager.

He added that Tamu Massif is different from classic seamounts, the
volcanoes that protrude off the ocean floor around the world by the tens
of thousands. Tamu Massif is much larger, with a much more gentle slope
than classic seamounts, Sager said.

Near the summit of Tamu Massif, the slope is only around one degree,
he said. Down the flank the slope is half a degree, and it’s even less
than that near the base. (The average slope of a staircase is 40
degrees, and an easy ski slope is about 10 degrees.) ”If you were
standing on the massif, you would have a hard time knowing which way is
down,” said Sager.

Finding an Unusual Structure

Scientists had thought the giant Shatsky Rise formed over time as a
composite of several volcanoes that grew together, in a process similar
to the way the big island of Hawaii was made by the outpourings of five
separate volcanoes that were in close proximity.

But when Sager and colleagues looked at seismic data of Tamu Massif, they were surprised at what they found.

“We saw what appear to be lava flows going out from the center of the
volcano in all directions, with no obvious large secondary source of
volcanism, so that was a surprise,” Sager said.

The team also performed geochemical analysis on core samples taken
from the massif. They found that the huge structure appeared to be made
out of the same rock, of the same age.

So the scientists concluded that Tamu Massif was created by a single
volcano, and probably over a relatively short period of time of a few
million years. The volcano went “extinct,” meaning inactive, shortly
after it formed, Saged added. That was probably in the Late Jurassic to
Early Cretaceous period, about 145 million years ago.

“If what they are saying is correct, that is truly a massive volcano,” said Brian Jicha,
a geologist at the University of Wisconsin who has received funding
from National Geographic to study the formation of the Aleutian Islands.

“There are a lot of these oceanic plateaus, so if some of them really
are just volcanoes, this paper might begin to change the way we think
oceanic plateaus are built, and maybe even some of the continental
basalt plateaus,” said Jicha, who was not involved in the study.

Sager agrees that more work is needed on other oceanic plateaus.
“There could be around a dozen of these things out there,” he said about
the possibility of more large shield volcanoes under the sea.

Sager noted that although Tamu Massif currently appears to be the
largest single volcano on Earth, there are still larger volcanic
complexes, such as the Siberian Traps,
which may hold other mysteries. Those features were likely made up of
molten rock from different sources, he said, unlike Tamu Massif's
formation according to the new theory.

How Did the Volcano Form?

Sager said scientists are still trying to work out the details of how Tamu Massif formed.

He said it seems likely that the spot on the seafloor had the right
mix of elements, including a boundary of three tectonic plates, thin
crust, and a source of hot magma below that was able to bubble up to the
surface. The molten rock poured out, and then built up a wide, gradual
rise as it cooled.
Precisely how the magma made it to the surface is an open question.
Perhaps a blob of the rock got superheated, and then rose to the surface
due to buoyancy. Or, cracks in the overlying crust could have opened,
allowing molten rock to spill out.

The next step will be more work to figure out what the source of the
magma was, said Sager. He would like to go back and measure the magnetic
properties of the rock, using a ship that is equipped with GPS. The
data will give him a better idea how the lava spread out, he said.

Jicha added that “if it is indeed really one volcano, and the case is
fairly compelling, the amount of magma that had to go through the
lithosphere [crust] is off the charts.”

“Not only does [Tamu Massif] give us a new wow in the form of a giant
new volcano, but it gives us new insight into a building block of an
oceanic plateau,” said Sager.

He’s not sure if the new volcano will help scientists better
understand Olympus Mons on Mars, but noted that “we can see the surface
of Mars better than we can see the bottom of the ocean.”

Tamu Massif, he said, “has been hiding out for 145 million years because it found a good place to hide.”